#define _CRT_SECURE_NO_WARNINGS 1

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head)
    {
        return dfs(nullptr, head);
    }

    ListNode* dfs(ListNode* prev, ListNode* head)
    {
        if (head == nullptr)return prev;
        ListNode* new_head_next = head->next;
        head->next = prev;
        return dfs(head, new_head_next);
    }
};